Introduces mechanism for background rfcomm servers

This change adds a mechanism in the AdapterService which allows it to
register RFCOMM listeners requested by a BluetoothAdapter.
This is so that applications can request the framework to listen for
incoming RFCOMM connections in the background.

Apps can request that the bluetooth manager, via a BluetoothAdapter,
listen on a specific service record, and then transact incoming socket
connections after receiving a notification of availability via a
PendingIntent.

Tag: #feature
Test: Run the CTS Bluetooth Rfcomm Handoff service Test. This requires
two devices.
Bug: 186494155
Ignore-AOSP-First: Need to commit to downstream first in order to
resolve a merge conflict.

Change-Id: Ia0c71969e691e6353f22fe3b7dae4a7500230e03
Merged-In: Ia0c71969e691e6353f22fe3b7dae4a7500230e03